In high frequency RF circuits, there are required a plurality of components, some components being active components and some being passive components. The passive components are comprised of reactive and passive components. The reactive components typically are comprised of capacitors and inductors whereas the passive components are typically resistors. However, when operating at high frequencies, the concept of “impedance” is utilized, which impedance typically is comprised of distributed inductance, distributed capacitance and distributed resistance. A simple conductor or line at DC will only have a resistive component. However, at high frequencies, there will be a series inductance associated with that line as well as a distributed capacitance between the line and any other conductor, the dielectric constant of the capacitor being the medium on which the line is formed.
One of the primary components in an RF circuit is an inductor, and one of the more difficult to fabricate. An ideal inductor at low frequencies is comprised of a coil that is either wound around a magnetic core or it is merely fabricated with a plurality of “turns” with the core being air. There will always be an inherent series resistance due to the wire utilized and, when wound about a core, there will be some magnetic loss in the core. Typically, if the inductor is freestanding, there will be very little capacitance coupling between the coil wire and adjacent bodies or conductors. Thus, the primary components of the inductor will be the series resistance and the number of turns of that inductor and the overall length of the wire used in the inductor. The resistance of the inductor has a direct correlation to the loss associated with that inductor. Of course, thicker wire can be utilized to reduce series resistance. However, this series resistance and/or the winding of the coil on the magnetic core, results in a decrease in “quality factor” or, as it is more commonly referred, the “Q,” especially at high frequencies. This Q-factor is a measure of the quality of the coil. If one wants to have a very sharp resonant circuit, it is desirable to have a very high Q-factor. This Q-factor directly relates to the loss of the coil. Thus, in high frequency circuits, it is desirable to have a very low loss coil, i.e., there should be minimal series resistance and there should be minimal capacitance between the turns of the coil and any adjacent conductors. Further, the medium that is disposed between turns of the coil should be, in the ideal, air.
In the first high frequency circuits, it was possible to fabricate the inductors as discrete components that could be soldered onto a circuit board. It was then possible to fabricate these coils around a very low loss core and utilize fairly low loss wire, resulting in a very high-Q coil with sufficient inductance. However, this was an expensive solution and it was desirable to fabricate the coils, if possible, on the substrate such that a resultant monolithic solution was achieved. Some of the first monolithic coils were those formed on thin film substrates such as quartz substrates. These coils typically took the form of a helical line pattern disposed on the quartz substrate beginning from a center point and spiraling outward therefrom to comprise the two terminals of coil. This resulted in fairly high Q-factor coils due to the fact that the dielectric constant of the quartz was fairly low. However, the size of the inductor was still restricted due to the amount of surface area required for the coil. If the line width was reduced, the series resistance went up and the Q-factor of the coil went down. Thus, these type of coils were limited to matching elements and, possibly, utilized for RF “chokes” which were required between a transistor terminal and a bias input. These chokes presented a high impedance to the circuit over a fairly narrow band frequencies, typically the operating band. Integrated circuits have seen a dramatic increase in speed thereof, resulting in the ability to fabricate integrated circuits operating upwards of 2-3 GHz. The need for monolithic matching elements, such as inductors and capacitors of high quality, has thus also increased. However, the problem with any type of inductor or capacitor is that it requires a certain amount of space, i.e., silicon surface area. Typically, there is the defined amount of surface area required for the inductor itself which is typically formed on one or two layers of the substrate structure with a “guard band” disposed thereabout to prevent unwanted coupling to other circuits. Typically, some type of ground plane or the such is required to be disposed between one RF component and another. The problem with these types of monolithic structures on a semi-conductor substrate is that they are typically fabricated on silicon dioxide. Thus, it is necessary to insure that the capacitance between any conductor in one of these reactive elements is minimized with respect to other conductors and that the series resistance is minimized. This series resistance is a function of the type of material from which the inductor is fabricated. Typically, these inductors will be fabricated in one or more of the metal layers, which metal is typically comprised of copper. Thus, any changes that can be made to an inductor to decrease the amount of space required for that inductor will be a desirable aspect of a monolithic RF inductor, as it will save valuable silicon real estate.